What is an ice and water shield membrane made of?
Short answer. An ice and water shield membrane is a layered sheet: a tough top surface of plastic film or fine granules, a thick core of rubberized asphalt or butyl adhesive, sometimes a fiberglass or polyester reinforcement, and a peel-off release liner on the back. It bonds to the roof deck, stays flexible, and closes around nail shanks, which is what makes it waterproof under shingles and metal.
Layer by layer
- Top surface. Polyethylene film, woven polypropylene or a mineral granule coating. It protects the adhesive, takes foot traffic and carries printed lap lines.
- Adhesive core. Most common is rubberized asphalt, asphalt modified with polymers such as SBS so it stays elastic. Butyl-based membranes are another family, often promoted for higher temperature tolerance.
- Reinforcement. Some products embed fiberglass or polyester for strength and tear resistance.
- Release liner. Silicone-coated plastic or paper that peels off, often split down the middle for easier positioning.
Why the adhesive matters
The adhesive is both the waterproofing and the bond. When a nail is driven through, the elastic compound grips the shank and closes the hole. That self-sealing behavior is what separates these membranes from nailed underlayments.
How membranes are tested
The common standard is ASTM D1970, which covers self-adhering underlayments used under steep roofing for ice dam protection. Testing includes properties such as nail sealability and flexibility at low temperatures. Look for that standard on the label or data sheet.
Surface types in practice
- Granular: better footing on steep roofs, rougher to handle.
- Film: lighter and smooth, but slick when wet or dusty.
- High temperature: formulated for metal roofs and hot applications.
How it compares with standard underlayment is covered in synthetic underlayment vs felt, and where it goes in ice and water shield in Georgia.